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Date:   Tue, 29 May 2018 13:58:11 +0000
From:   Christopher Lameter <cl@linux.com>
To:     Vladimir Davydov <vdavydov.dev@gmail.com>
cc:     Shakeel Butt <shakeelb@google.com>,
        Michal Hocko <mhocko@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Greg Thelen <gthelen@google.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Tejun Heo <tj@kernel.org>, Linux MM <linux-mm@kvack.org>,
        cgroups@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] mm: fix race between kmem_cache destroy, create and
 deactivate
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On Sat, 26 May 2018, Vladimir Davydov wrote:

> > The reference counting is only implemented for root kmem_caches for
> > simplicity. The reference of a root kmem_cache is elevated on sharing or
> > while its memcg kmem_cache creation or deactivation request is in the
> > fly and thus it is made sure that the root kmem_cache is not destroyed
> > in the middle. As the reference of kmem_cache is elevated on sharing,
> > the 'shared_count' does not need any locking protection as at worst it
> > can be out-dated for a small window which is tolerable.
>
> I wonder if we could fix this problem without introducing reference
> counting for kmem caches (which seems a bit of an overkill to me TBO),
> e.g. by flushing memcg_kmem_cache_wq before root cache destruction?

Would prefer that too but the whole memcg handling is something of a
mystery to me.